Written by: Dr. Jacquie Jacob, University of Kentucky

There has been some confusion lately about the nature and safety of colored eggs, such as the blue-shelled eggs produced by Araucana chickens.

Araucana eggs
Eggs from Araucana chickens (Image by Bartosz Luczak on


The blue and green eggs that are produced by certain chicken breeds are due to the species’ genes. The Virology Blog published an article, on September 11, 2013, about the evolution of the blue trait in chicken eggshells. The article stated that the blue color is the result of a gene inserted into the chicken genome by a retrovirus. This was followed up to an August 28, 2013, Food Safety News article. The information in these articles has led some readers to believe that Araucana chickens lay blue-shelled eggs because they are sick. Others interpreted the information to mean that Araucana chickens are genetically modified organisms (GMOs), and the eggs they produce should be labeled as such. Neither is correct.


The purpose of the Virology Blog was to highlight the importance of viruses in “shaping evolution and diversity of species.” A retrovirus is a virus with RNA instead of the DNA that animal chromosomes have. Because it has no DNA, a retrovirus cannot reproduce on its own. Instead, it uses an enzyme that chemically transforms its RNA into DNA and inserts this small amount of DNA material into the infected cell. This is a natural phenomenon that has been occurring since the beginning of life. In fact, about 8% of human DNA came about this way. With Araucana chickens, and other breeds laying colored eggs, the genetic code left behind by the retrovirus many generations ago resulted in blue-shelled eggs. The chickens are not spreading the virus, but rather, the virus is just a harmless part of their genetic story.

Bantam female white Araucana chicken
Bantam female white Araucana chicken (Image by Dr. Jacquie Jacob, University of Kentucky)


How viruses work.